1. Field of the Invention
The present invention relates to a circuit configuration for controlling an element to be driven, such as an electroluminescence display element.
2. Description of Related Art
Electroluminescence (EL) display apparatuses using, as an emissive element, a self-emissive EL element in each pixel are advantageous in that they are self-emissive, are thin, and consume a small amount of power. Therefore, EL display apparatuses have attracted interest and have been studied as potential replacements for display apparatuses such as CRT or LCD displays.
In particular, there is anticipation that active matrix EL display apparatuses in which a switching element, such as a thin film transistor (TFT), for individually controlling the EL element is provided for each pixel to thereby control the EL element for each pixel will become available as high resolution display apparatuses.
FIG. 1 illustrates a circuit configuration of each pixel in an active matrix (including m rows and n columns) EL display apparatus. In the EL display apparatus, on a substrate, a plurality of gate lines GL extend in the row direction and a plurality of data lines DL and drive power source lines VL extend in the column direction. Each pixel includes an organic EL element 50, a switching TFT (first TFT) 10, a TFT (second TFT) 21 for driving the EL element (hereafter referred to as an element-driving TFT) and a storage capacitor Cs.
The first TFT 10 is connected with the gate line GL and the data line DL, and turns ON when a gate signal (a selection signal) is applied to the gate electrode of the TFT 10. At this time, a data signal supplied to the data line DL is stored in the storage capacitor Cs which is connected between the first TFT 10 and the second TFT 21. A voltage in accordance with the data signal, supplied via the first TFT 10, is applied to the gate electrode of the second TFT 21, which then supplies a current in accordance with the applied voltage value from the power source line VL to the organic EL element 50. In the organic EL element 50, holes injected from the anode and electrons introduced from the cathode are recombined in the emissive layer, to thereby excite emissive molecules. Through the process in which these emissive molecules excite until deactivation, the organic EL element 50 projects light. The emission brightness of the organic EL element 50 is substantially proportional to the current supplied to the organic EL element 50. Therefore, by controlling the current to be supplied to the organic EL element 50 in accordance with a data signal for each pixel as described above, the organic EL element is caused to emit light of a brightness corresponding to the data signal, so that a desired image is displayed by the display apparatus as a whole.
In such an organic EL display apparatus, in order to achieve high display quality, it is necessary to cause the organic EL element 50 to reliably emit light at a brightness corresponding to a data signal. Accordingly, for the active matrix type EL display apparatus, it is required that the drain current does not change in the second TFT 21 which is disposed between the drive power source line VL and the organic EL element 50, even when the anode potential of the organic EL element 50 changes due to a current flowing through the EL element 50.
For this reason, as shown in FIG. 1, for the second TFT 21 is often adopted a p-channel TFT in which the source is connected with the drive power source line VL, the drain is connected with the organic EL element 50 on the anode side, and the source-drain current can be controlled by a potential difference Vgs between the source and the gate to which a voltage in accordance with a data signal is applied.
When a p-channel TFT is employed as the second TFT 21, however, there is a problem that a voltage change of the drive power source line VL causes a change in the emission brightness of each element 50, because in the p-ch TFT the source is connected with the drive power source line VL and the drain current, namely a current to be supplied to the organic EL element 50, is controlled by a potential difference between the source and the gate, as described above. Because the organic EL element 50 is a driven-by-current type element as described above, when a bright image is displayed for a certain frame period (when, for example, a large white area is displayed), for example, a great amount of current flows at a time from a single drive power source Pvdd to a large number of organic EL elements 50 on the substrate via the corresponding drive power source lines VL, and the potential of these drive power source lines VL changes. Further, in a region which is far from the drive power source Pvdd and has a significant voltage drop due to line resistance of the drive power source line VL, such as in a pixel positioned distant from the power source, the drive power source line VL at a low voltage results in the emission brightness of each organic EL element 50 being lower than that of elements located closer to the power source.
In addition, when a p-ch TFT is used as the second TFT 21, it is necessary to reverse the polarity of a data signal to be supplied to the second TFT 21 with regard to the polarity of a video signal, and thus necessary to provide a polarity reverse means in the driver circuit.